1. Field of the Invention
The present invention relates to an LED (Light Emitting Diode) driving device, an illuminating device using an LED as its light source, and a projection-type display device.
2. Description of the Related Art
A field-sequential display device is an example of projection-type display devices, and forms a color image by time-divisionally displaying R (red), G (green), B (blue). The formation of a color image in an exemplar DLP (Digital Light Processing) projector is performed by: employing a high-pressure mercury lamp or the like as a light source; separating the white light from the light source into colors by means of a color wheel; modulating the color-separated light by means of a reflective device such as a DMD (Digital Micromirror Device); and then projecting the modulated light on a screen through a projection optical system.
The quantization noise causes a problem in low-intensity display of the brightness expression implemented by a display device with such a reflective device. To address this problem, a conventional display device employing a lamp such as a high-pressure mercury lamp is equipped with an ND (Neutral Density) filter attached to the segments of the color wheel. The ND filter is designed to lower the intensity down to approximately 10% so that the apparent bits in low-intensity display can be increased and thus the quantization noise can be reduced.
Another example of field-sequential display devices employs LEDs of RGB colors as light sources in place of a white-lamp light source with a color wheel. The RGB LEDs emit light in a time-dividing manner, and the light thus emitted enters a reflective device to be modulated. The resultant light is then projected on a screen through a projection optical system to form a color image. Note that, in this case, the light emission for each LED is turned on and off by pulsing.
Meanwhile, liquid-crystal displays are examples of direct-view display devices. The light source of the liquid-crystal display has come to employ solid-state illumination (i.e., LEDs) in place of fluorescent tubes. An improvement in the performance of the liquid-crystal display has been achieved by a technique (known as an area-active technique). In the technique, the intensities of the multiple LEDs that the liquid-crystal display device is equipped with are changed for such groups of LEDs as determined in accordance with the video image to be displayed by the liquid-crystal display device. The visual dynamic range is thus changed resulting in the above-mentioned improvement in the performance.    [Patent Document 1] JP-A-2001-313423    [Patent Document 2] JP-A-2002-203988    [Patent Document 3] JP-A-2004-274872    [Patent Document 4] JP-A-2005-142137